Thin-film transistor (TFT) liquid crystal displays (LCDs) have been widely used in areas such as personal notebook computers, portable televisions and video camera monitors. In conventional TFT LCDs, the back-light consumes most of the power. In order to reduce the power consumption of the LCD devices, there are two approaches commonly adopted. One is improving the efficiency of the back-light and the other is increasing the transmission of the light through the liquid crystal cells.
Many different technologies of manufacturing TFT-LCD devices with various structures have been presented to improve the back-light efficiency. U.S. Pat. No. 5,478,611 of T. Hashimoto discloses a glass substrate for LCDs in which unnecessary light of the region other than pixel electrodes can be completely shielded, the TFT optical leak current can be totally suppressed, and the light reflection at the black matrix portion can be almost fully depressed. M. Katayama et. al. presented "A 10.4 in. Diagonal Full-Color TFT-LCD with New Self-aligned a-Si TFTs for OHP System" in pages 243-246 of Conference Record of the 1991 International Display Research Conference. The light degradation of the LCD performance has been suppressed by new self-aligned a-Si TFTs and a new driving method presented in the article.
The light transmission through the liquid crystal cells depends on the aperture ratio of a TFT-LCD device. Much research and development work has been dedicated to obtaining a high aperture ratio for TFT-LCD devices. N. Takahashi presented a structure that achieves a 35% aperture in "A High-Aperture Ratio Pixel Structure for High-Density a-Si TFT Liquid-Crystal Light Valves" of SID 1993 DIGEST, pp. 610-613. "A High-Aperture TFT-LCD with Shield-Electrode Structure" of SID 1993 DIGEST, pp. 739-742 presented by T. Ueda shows that a 58% aperture has been developed. T. Kitazawa et. al. further shows an ultra-high-aperture ratio of 70% by using a shield-electrode structure and a storage-capacitor-on-gate structure in "A 9.5-in. TFT-LCD with an ultra-High-Aperture-Ratio Pixel Structure" of SID 1994 DIGEST, pp. 365-368. An analytical investigation of the aperture ratio on high TFT-array structure has been discussed by K. Suzuki in "High-Aperture TFT Array Structures" of SID 1994 DIGEST, pp. 167-170.
It is clear that there is a strong need in achieving a high aperture ratio for the TFT-LCD devices. Various designs and techniques in forming the black matrix structure in TFT-LCDs have been studied to increase the aperture ratio and improve the back-light efficiency.